[llvm] [RegisterCoalescer] Fix subrange update when rematerialization widens a def (PR #151974)

[via llvm-commits]

https://github.com/KRM7 created https://github.com/llvm/llvm-project/pull/151974

None

>From d1371d345e8d77eeed55c7119e896e6373f3f2ea Mon Sep 17 00:00:00 2001
From: Krisztian Rugasi <Krisztian.Rugasi at hightec-rt.com>
Date: Mon, 4 Aug 2025 15:58:22 +0200
Subject: [PATCH] [RegisterCoalescer] Fix subrange update when
 rematerialization widens a def

---
 llvm/lib/CodeGen/RegisterCoalescer.cpp        | 10 ++++----
 .../regcoal-subranges-update-remat.mir        | 25 +++++++++++++++++++
 2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index 2d7987a2e1988..514f2f02d6425 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -1624,11 +1624,11 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
           UpdatedSubRanges = true;
         } else {
           // We know that this lane is defined by this instruction,
-          // but at this point it may be empty because it is not used by
-          // anything. This happens when updateRegDefUses adds the missing
-          // lanes. Assign that lane a dead def so that the interferences
-          // are properly modeled.
-          if (SR.empty())
+          // but at this point it might not be live because it was not defined
+          // by the original instruction. This happens when the
+          // rematerialization widens the defined register. Assign that lane a
+          // dead def so that the interferences are properly modeled.
+          if (!SR.liveAt(DefIndex))
             SR.createDeadDef(DefIndex, Alloc);
         }
       }
diff --git a/llvm/test/CodeGen/SystemZ/regcoal-subranges-update-remat.mir b/llvm/test/CodeGen/SystemZ/regcoal-subranges-update-remat.mir
index e3207df799449..d9fe810e23b3a 100644
--- a/llvm/test/CodeGen/SystemZ/regcoal-subranges-update-remat.mir
+++ b/llvm/test/CodeGen/SystemZ/regcoal-subranges-update-remat.mir
@@ -43,3 +43,28 @@ body:             |
     %3:gr32bit = COPY killed %1
     Return implicit %3
 ...
+
+---
+name:            test_dead_at_remat_later_defined
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    ; CHECK-LABEL: name: test_dead_at_remat_later_defined
+    ; CHECK: undef [[LHI:%[0-9]+]].subreg_l32:gr128bit = LHI 0
+    ; CHECK-NEXT: [[LHI:%[0-9]+]].subreg_l64:gr128bit = LGHI 2
+    ; CHECK-NEXT: [[LHI1:%[0-9]+]]:gr32bit = LHI 1
+    ; CHECK-NEXT: [[LHI:%[0-9]+]].subreg_lh32:gr128bit = COPY [[LHI1]]
+    ; CHECK-NEXT: [[LGHI:%[0-9]+]]:gr64bit = LGHI 2
+    ; CHECK-NEXT: [[LHI:%[0-9]+]].subreg_h32:gr128bit = COPY [[LGHI]].subreg_l32
+    ; CHECK-NEXT: $r0q = COPY [[LHI]]
+    ; CHECK-NEXT: $r4d = COPY [[LGHI]].subreg_h32
+    %0:gr64bit = LGHI 2
+    %1:gr32bit = LHI 0
+    %2:gr32bit = LHI 1
+    undef %3.subreg_ll32:gr128bit = COPY %0.subreg_l32
+    %3.subreg_lh32:gr128bit = COPY %2
+    %3.subreg_l32:gr128bit = COPY %1
+    %3.subreg_h32:gr128bit = COPY %0.subreg_l32
+    $r0q = COPY %3
+    $r4d = COPY %0.subreg_h32
+...